1. Field of the Invention
The invention relates to a vehicle control apparatus and a vehicle control method for a vehicle having an engine with a fuel injection valve and an automatic transmission with a torque converter having a lock-up clutch.
2. Description of the Related Art
Recently, toque converters provided in automatic transmissions are often provided with a lock-up clutch via which the input side (pump impeller side) and the output side (turbine runner side) of the torque converter are directly coupled with each other when needed.
More specifically, when the lock-up clutch is applied, the input side (pump impeller side) and the output side (turbine runner side) of the toque converter are directly coupled with each other. When the lock-up clutch is released, on the other hand, the input side and the output side of the toque converter are decoupled from each other. Further, in some applications, “slip control” is performed in which the lock-up clutch is placed in a semi-applied state, which is a state between the applied state and the released state, so that the lock-up clutch slips.
This slip control is started in response to a predetermined condition (e.g., a condition defined by the vehicle speed and the accelerator opening) coining into effect. During the slip control, the power transfer state at the torque converter is controlled through feedback control that is executed based on the difference between the rotation speed of the pump impeller of the toque converter (corresponding to the engine speed) and the rotation speed of the turbine runner of the torque converter such that the application force of the lock-up clutch equals a required force.
Example states where the above-described slip control is executed will be briefly described in the following.
In general, when the vehicle is decelerating, fuel injection from the fuel injection valves is suspended in order to improve the fuel economy, and such fuel injection suspension is generally called “fuel-cut”. Fuel-cut is normally canceled when the engine speed has decreased to or below a predetermined threshold.
If the lock-up clutch of the torque converter is in the released state when the vehicle is decelerating as described above, it causes a loss of torque (rotational force) transferred from the output side to the input side of the torque converter, resulting in a decrease in the engine brake force. In such a case, therefore, the engine brake force is increased by maintaining the lock-up clutch in the applied state.
If the fuel-cut is started as the vehicle continues to decelerate in the state described above, due to the increased engine brake force, the engine speed and the vehicle speed sharply drop, and the engine speed reaches the threshold for canceling the fuel-cut within a relatively short time, and therefore the fuel economy improving effect by the fuel-cut is low.
One option to cope with this issue may be to extend the fuel-cut duration by lowering the threshold for canceling the fuel-cut. However, if the threshold is lowered excessively, it increases the possibility of engine stall and therefore it is not desirable.
In view of this, technologies have been proposed in which the slip control of the lock-up clutch is executed during the fuel-cut so as to slow the decrease in the engine speed so that it takes longer for the engine speed to decrease down to the threshold for canceling the fuel-cut (For example, refer to Japanese patent application publications No. 06-174074 (JP-A-06-174074), No. 06-193491 (JP-A-06-193491), and No. 2005-009510 (JP-A-2005-009510)).
Moreover, technologies have been proposed in which, in order to further extend the fuel-cut duration, so-called coast-downshift control is executed during the slip control of the lock-tip clutch (For example, refer to Japanese patent application publications No. 2003-074695 (JP-A-2003-074695) and No. 2000-154742 (JP-A-2000-154742)). In the coast-downshift control, the automatic transmission is downshifted at an engine speed (coast-downshift threshold) that is set slightly higher than the threshold for canceling the fuel-cut, so that the fuel-cut duration extends.
When the transmission speed to which the automatic transmission is to be downshifted as a coast-downshift is lower than a coast-downshift lower limit transmission speed that is set in advance, execution of the coast-downshift is prohibited and the fuel-cut and the slip control of the lock-up clutch are discontinued.
The coast-downshift lower limit transmission speed is set to a relatively high transmission speed to prevent the engine brake force from becoming too large. That is, the coast-downshift is limited when the fuel-cut and the slip control are being executed.
Thus, because the decrease in the engine speed is slowed by the slip control of the lock-up clutch and the coast-downshift threshold is set higher than the fuel-cut cancellation threshold, the fuel-cut continues even after the automatic transmission is downshifted. As such, the fuel-cut duration extends and the fuel economy improves accordingly.
Although it is true that the control method described above extends the fuel-cut duration during deceleration of the vehicle, it still has room for further improvements as follows.
For example, when the vehicle is traveling on a downhill road, not a flat road, the larger the downhill gradient of the downhill road, the smaller the deceleration of the vehicle becomes, and even in this case, after the automatic transmission has been downshifted to the coast-downshift lower limit transmission speed during the coast-downshift control, the fuel-cut and the slip control of the lock-up clutch are discontinued and the fuel supply resumes, which eliminates the feeling of deceleration and reduces the fuel improving effect.